The present invention relates to a retarding field spectrometer and, more particularly, to a spectrometer for use with a scanning electron microscope (SEM) or the like.
Considerable literature exists describing the use of scanning electron microscopes for topographical voltage and waveform determinations over the surface of an integrated circuit. One such article by H. P. Feuerbaum entitled "VLSI Testing Using the Electron Probe" was published in Scanning Electron Microscopy/1979/I, SEM Inc., AMF O'Hare, Ill. 60666, 285-296, 318. As noted in the "Abstract" at the beginning of said article, the electron beam probe is an extraordinarily valuable aid for the developer of VLSI circuits because its almost negligible load, non-destructive operation and easy positioning allows the measurement of voltage at any desired point of an integrated circuit. However, the article observes that the measurement of voltages in the sampling mode on a 64 k MOS-RAM requires a reduction of the beam diameter to 1.2 micrometers (2.5 kV, 10.sup.-7 A) and an improvement of the voltage resolution to below the 10 mV mark. In order to accomplish such improvement the article explains that a retarding field spectrometer, developed for the purpose, and having a height permitting a working distance of 5 mm, should be used. The author describes an experimental set-up in which the beam is generated by the electron microscope column of an ETEC AUTOSCAN SEM. The spectrometer assembly includes an extraction electrode in the form of a screen maintained at 600V. This electrode is mounted about 1.1 mm above the target or sample surface oriented with its plane normal to the beam axis. Approximately 4 mm above the extraction electrode is mounted a retarding field electrode maintained at zero potential. Approximately 4.6 mm above the latter electrode is a further electrode maintained at -5V and this is spaced about 0.5 mm below the recessed pole piece of the microscope column. To one side of the space between the last two mentioned electrodes is a deflection field electrode maintained at 120V. In the overall system, the target is scanned by a pulsed primary beam and the secondary electrons having sufficient energy to get by the retarding field electrode are accelerated laterally to impinge upon a scintillator photomultiplier whose output is amplified and ultimately supplied to an oscilloscope.
In general, the article explains that present IC's have metal lines that are 4 micrometers wide, and to measure the voltage on the same the beam diameter should not exceed 1.2 micrometers. However, the article observes that circuits with much narrower lines are in development and that a future width of 1 micrometer is to be expected. But to measure the voltage on such lines the beam diameter will have to be reduced to about 0.3 micrometers. The article speculates that this can be accomplished by replacing the existing beam source with "an LaB.sub.6 or field emission gun."
In the application of David Fairhurst Lewis entitled: "A Memory Element for Information Storage and Retrieval System and Associated Method," Ser. No. 047,350, filed May 8, 1987, and assigned to the same assignee as the present invention, there is described a recording medium that is used as a memory element on which information is written and read through irradiation with an electron beam. The invention disclosed and claimed in said application enables addresses to be located in the memory element with sufficient precision that in excess of 10.sup.8 bits/cm.sup.2 can be recorded and retrieved. As explained in the Lewis application, to achieve such density the writing beam must be positionable and re-positionable with an accuracy in the neighborhood of about 0.1 micrometers. In addition, said application describes examples establishing that information can be recorded at a density on the order of 10.sup.9 bits/cm.sup.2.
While the recording medium described and claimed in said Lewis application provides for addressing recording areas with the precision necessary for such high density recording, making maximum use of such recording medium requires significantly more sensitive beam writing and reading equipment than that heretofore available. In particular, where writing is accomplished by depositing a charge pattern on an insulating substrate, high resolution reading requires the ability to precisely direct the reading beam onto the medium and accurately detect returning secondary electrons while discriminating as to the energies of such secondaries. Sensitive beam writing is also important, and while the present invention is able to provide for the necessary sensitivity, sensitive beam writing can be enhanced by employing the procedure disclosed in the U.S. patent application Ser. No. 787,946 of Henry Seiwatz, filed Oct. 16, 1985 and entitled "Reduction of Deflection Errors in E-Beam Recording".